1. Field of the Invention
The present invention relates to a heating resistor, a heating resistor for use in a ceramic heater, and a ceramic heater using the heating resistor. More particularly, the present invention relates to a heating resistor having excellent heat resistance and being useful in applications where oxidation thereof raises no problem; a heating resistor for use in a ceramic heater to be used in special applications requiring heating, such as a water heater and a glow plug of a diesel engine; and a ceramic heater which includes the heating resistor for use in a ceramic heater embedded in a substrate thereof.
2. Description of the Related Art
Conventionally, there has been known a highly heat-resistant sintered silicon nitride compact having in grain boundaries a crystal phase which is composed of, for example, disilicate having a high melting point, or melilite having an even higher melting point. Also, in manufacture of a heating resistor composed of conductive ceramic such as WC, MoSi.sub.2, or TiN and silicon nitride ceramic, there is obtained a sintered compact which has a glass phase in grain boundaries thereof as a result of the use of a sintering aid such as MgO or an Al.sub.2 O.sub.3 --Y.sub.2 O.sub.3 system, which sintering aid forms a liquid phase at relatively low temperature. Such a sintered compact has been used in various applications.
In a ceramic heater having a heating resistor embedded in a substrate thereof, the surface temperature of the heater becomes considerably high while the ceramic heater is being energized. Particularly, in a high-temperature-type glow plug, the surface temperature rises to about 1400.degree. C., and the temperature of a portion of the interior rises to 1500.degree. C. or higher.
The aforementioned sintered compact having melilite in grain boundaries has excellent mechanical strength at high temperature, but involves disadvantages in that oxidation tends to occur at relatively low temperature and that cracking may arise from volume expansion. In a sintered compact having a crystal phase such as a melilite phase or a disilicate phase, the crystal phase has a high melting point, but, in the case of a certain composition, a vitreous substance of relatively low melting point remains in grain boundaries in a small amount. As a result, even when such a crystal phase is precipitated in a heating resistor which contains a conductive component, the vitreous substance conceivably causes impairment in working durability. Such impairment has not yet been verified.
In manufacture of a heating resistor, when a sintering aid, such as MgO or an Al.sub.2 O.sub.3 --Y.sub.2 O.sub.3 system, which forms a liquid phase at relatively low temperature is used, the sintering aid remains in the form of a glass phase of low melting point in the grain boundaries of silicon nitride ceramic after firing is completed. The remaining glass phase impairs mechanical strength at high temperature and working durability of a heating resistor and a ceramic heater using the heating resistor. Particularly, in the case of a high-temperature-type glow plug, which involves a temperature rise to 1500.degree. C. or higher at a certain portion thereof, a heater may break due to impairment in strength, or a heating resistor may break due to decomposition of the glass phase in grain boundaries caused by a potential difference induced during application of electricity. In order to suppress the decomposition, there has been proposed a method in which nitrogen is introduced into the grain boundaries so as to increase viscosity of the grain boundaries through formation of oxynitride glass. However, the method fails to solve the problem sufficiently.